A light amplifier is generally designed so that it is necessary to suppress a light surge as extremely as possible. The "light surge" used herein means a phenomenon in which signal light of very high gain is outputted from the light amplification apparatus when a signal light input to the light amplification apparatus increases instantaneously. Such a light surge occurs for the following reason. That is, when a signal light input decreases, it is necessary to enlarge the power of excitation light to increase the amplification factor in the light amplification medium in order to obtain a desired light output. Accordingly, in this occasion, large signal light amplification energy is potentially accumulated in the inside of the light amplification medium. If the signal light input increases instantaneously in this state, the signal light suffers the accumulated energy so that the signal light of a very high gain is emitted from the light amplification apparatus. Because the occurrence of a light surge not only brings the breaking of a light-receiving element and the induction of melting of a connector end surface on the light communication terminal side but also brings human trouble (visual trouble) according to circumstances, it is necessary that the occurrence of the light surge is suppressed as extremely as possible. Particularly, when a plurality of light amplification apparatuses are connected multistageously, the situation is more serious. This is because the once generated light surge is amplified successively by the connected light amplification apparatuses so that the amplified light surge may give a death-blow to optical parts constituting each of the light amplification apparatuses.
Heretofore, a measure counter to such a light surge has been described in the paper "Discussion of Light Surge in Multistageous Connection of Light Amplifiers" (Spring Meeting B-941, Institute of Electronics, Information and Communication Engineers of Japan, 1993). FIG. 14A shows the configuration of an experimental system in the countermeasure, and FIG. 14B shows the levels of light outputs from the respective light amplification apparatuses in the case of multistageous connection. As shown in FIG. 14A, the system is configured so that signal light with variable rising time can be emitted from a laser diode (LD) (using a distribution feedback type (DFB) LD module with a mean wavelength of 1.55 .mu.m) as a signal source by current-driving the LD. The signal light from the LD is successively given to light amplification apparatuses (each having an erbium-containing optical fiber amplifier excited by using a pump laser with a wavelength of 1.48 .mu.m) AMP1 to AMP5 provided with light attenuators (ATT) disposed prior to the light amplification apparatuses respectively, so that light outputs are obtained from the light amplification apparatuses respectively. On the other hand, the waveform states of the signal light outputs from the light amplification apparatuses can be monitored by a photodiode (PD) through an ATT. It is apparent from FIG. 14B that the light surge can be suppressed as the rising time of the signal light from the LD is elongated and that there is little light surge generated particularly when the rising time is set to be in a value of the order of milliseconds.
In the aforementioned paper, however, only a light surge suppressing method using the rising time control of the signal light input has been disclosed. Accordingly, the application of the light surge suppressing method to a practical light communication system cannot but be limited considerably so that, for example, the method cannot be applied to light surges caused by factors other than the rising of the signal light input. Even in the case where the rising time of the signal light input is controlled, there is a risk that a light surge is induced easily with the instantaneous change of the signal light power caused by the physical vibration or shock given to optical fiber in a light signal transmission state if the physical vibration or shock is given to the optical fiber.
Further, in addition to the aforementioned disadvantage, in order to suppress the light surge in each light amplification apparatus in the prior art, it is necessary to decrease the power of excitation light from the excitation (light) source or to stop the excitation light once. In this occasion, the light surge immediately follows the excitation light power reducing speed so that the light surge cannot be controlled so as to be suppressed. Accordingly, improvement in control response cannot be expected. This is because the light surge suppressing speed is slower than the excitation light power reducing speed though the degree of light surge suppression depends on energy accumulated before the rising of the signal light inputted to the light amplification medium, the rising speed of the signal light and the light power thereof. Accordingly, the excitation light output from the excitation source is stopped temporarily unless the light surge is suppressed to a predetermined value. In other words, this means that there is some vacant period in which the light surge, in fact, cannot be controlled so as to be suppressed effectively only by the excitation source and that the light surge is generated continuously in the vacant period.
Further, in the light amplification apparatus in the prior art, it is necessary to change widely the drive current given to the excitation source so that the light output from the light amplification apparatus is stabilized against the instantaneous change of the signal light input. In the case where the driving current is changed widely, the change in oscillation wavelength of the excitation source causes a failure in stability of the light output and deterioration of S/N in the whole of the light amplification apparatus.